During the last year, we continued to define the molecular events that regulate hematopoietic stem and progenitor cell (HSPC) quiescence, survival, self-renewal and, myeloid cell lineage commitment and differentiation. We have focused our efforts on transcription factors since they are essential for stem cell growth and differentiation, and are frequently deregulated during the development of leukemias and lymphomas. We previously found that the helix loop helix (HLH) transcription factor, inhibitor of differentiation 1 (Id1), is induced during the early stages of myeloid development, and can instruct hematopoietic stem cells toward a myeloid versus erythroid and lymphoid cell fate suggesting that this gene and other family members Id2 and Id3 may regulate cell specification from HSPC. We have also found that Id1 is required for normal hematopoietic development using Id1-/- mice. Id-/- mice are viable and show no obvious defects. However, these mice have hematopoietic phenotypes including increased hematopoietic stem/progenitor cell cycling and defects in B cell and myeloid cell development. We determined that the hematopoietic phenotype observed in the Id1-/- mice was not intrinsic to the Id1-/- hematopoietic cells, but were due to defects in the microenvironment or niche. The hematopoietic microenvironment is a complex mixture of cells that includes endothelial cells, mesenchymal stem and progenitor cells and their differentiated progeny, adipocytes, osteoblasts, smooth muscle, and chondrocytes. Therefore, we developed and tested a conditional mouse model of Id1, and have initiated studies to specifically delete Id1 in endothelial cells, osteoblasts and other stromal cell populations using an Id1 conditional mouse model recently developed in our lab, and transgenic mouse strains that express cre recombinase in endothelial, adipocytes and osteoblasts cell lineages. Since Id genes are known to compensate for each other, we evaluated if Id3 compensates for loss of Id1 by deleting both genes intrinsically and extrinsically using conditional mouse models and transgenic mice that express cre in specific cell lineages. We found that deletion of Id1 and Id3 in the hematopoietic microenvironment lead reduced overall hematopoietic output and extra medullary hematopoiesis, while BMC that lacked Id1 and Id3 developed normally when transplanted into recipient mice. Unexpectedly, we discovered that mice that lacked Id1 or Id1 and Id3 showed enhanced HSC self-renewal compared to WT BMC, and are currently pursuing molecular mechanism(s) that contribute to this phenotype. We previously discovered that Id2 is a physiological regulator of B cell and erythroid cell development by negatively regulating functions of E2A and Pu.1 respectively. We have found that over expression of Id1, Id2 and Id3 in HSPC results in a myeloproliferative disease (MPD) in mice transplanted with transduced HSPC. In addition, we determined that Id1 and Id2 are over expressed in many AML patient samples suggesting a role for these genes in hematopoietic malignancies. We have recently determined that over expression of Id1 promote, and knock down of Id1 inhibits the growth of AML cell lines in vitro and in vivo suggesting that Id1 may represent a viable therapeutic target to treat hematopoietic malignancies. In an effort to identify novel transcriptional regulators of myeloid cell growth and differentiation, we have compared the global gene expression profile of undifferentiated and differentiating hematopoietic progenitor cells. We have identified a novel zinc finger transcription factor of unknown function, POGZ, which is down regulated during the early stages of myeloid development. We have generated a mouse strain with a targeted deletion of POGZ. POGZ-/- mice do not survive beyond the first few hours of life, and die at birth of unknown causes, suggesting that POGZ is essential for mouse survival. We have discovered that fetal liver hematopoietic cell development is impaired in POGZ-/- mice, which includes a dramatic decrease in cellularity. We have found that loss of Pogz expression leads to a decrease in Bcl11a and KLF2 expression, and that embryonic globin genes are not silenced. We have found that Pogz directly regulates Bcl11a gene expression via a recently discovered erythroid specific enhancer element in the second intron of Bcl11a, and that POGZ is bound to the Bcl11a enhancers using ChIP assays. Gain and loss of POGZ expression studies have confirmed that POGZ is in a pathway that regulates Bcl11a gene expression. We have found that fetal globin expression is not repressed in mice transplanted with Pogz knockout fetal liver cells, demonstrating that POGZ is intrinsically required to repress fetal globin genes in vivo. Future studies are focused on confirming that POGZ is intrinsically required to repress fetal globin gene expression using Epor-Cre and Mx1-cre transgenic mouse models.